This invention relates to a lubricant composition useful for producing a magnetic recording medium, etc., and a magnetic recording medium obtained by use of said lubricant composition.
In the field of magnetic recording, improvements, such as digitization, miniaturization, long-time running, etc. are in progress, so that there is a growing demand for enhancement of the density of recording media. In particular, thin metal film type recording media having a magnetic recording layer composed of a thin ferromagnetic metal layer have been vigorously investigated in recent years because they are very advantageous for recording at a short wavelength.
In such circumstances, lubricants for magnetic recording medium are also noted and it has turned out that the performance characteristics of a magnetic recording medium itself can be improved by improving the lubricant.
For example, carboxylic acid esters have been known as the lubricants for magnetic recording medium. However, they involve, for example, the following problem: particularly when they are stored at high temperature and humidity, water repellency of the lubricant layer is reduced, and thus water tends to penetrate into the lubricant layer, as a result of which adhesion of the lubricant is weakened and the lubricant itself is coagulated, and hence the original characteristics of the lubricant cannot be maintained any longer.
Fluorocarboxylic acids and esters derived there-from involves, for example, the following problems: the fluorocarboxylic acids damage the surface of a magnetic recording layer or the surface of a protecting layer constituting the magnetic recording medium because they are strong acids; the derived esters are easily hydrolyzable and are not sufficient in weather resistance; synthesis of useful fluorocarboxylic acids having a large number of carbon atoms is difficult; and fluorocarboxylic acids having a relatively small number of carbon atoms and esters derived therefrom are not sufficient in friction-reducing effect, namely, they impart only insufficient running durability.
Ester type lubricants obtained from a mono-carboxylic acid and a perfluoroalkyl alcohol (JP-A 62-256218) have a relatively small cross sectional area of the molecule because of a polar group, a carbonyl group, in its molecule. Therefore, they are advantageous in that they can easily cover the surface of a magnetic layer or the surface of a protective layer constituting a magnetic recording medium. They, however, are disadvantageous in that sufficient exhibition of their lubricating effect is difficult because the alkyl group in the molecule is oriented in such a way as being inclined to the surface of the magnetic layer or protective layer.
Ester type lubricants obtained from a dicarboxylic acid and a fluoroalkyl alcohol (JP-A 5-128498, JP-A 5-282662 and JP-B 6-28717) are advantageous in that since molecules of the lubricant are adsorbed and oriented on the surface by two polar groups per molecule, the alkyl group in the molecule is hardly oriented in such a way as being inclined to the surface, so that the lubricant easily exhibits its lubricating effect. On the other hand, the lubricants are disadvantageous in that the cross sectional area of the lubricant molecule becomes relatively large because of the two polar groups and hence it becomes difficult to cover the surface by the lubricant molecule. In addition, when cooled to room temperature after storage at high temperature and humidity, the lubricants are recrystallized in some cases because of their strong tendency to molecular orientation, which causes deterioration of the weather resistance of the magnetic recording medium. Moreover, the lubricants are disadvantageous also in that the molecular weight of the lubricants is rather high due to their structure, and thus in magnetic recording mediums using these lubricants, still characteristics are lowered at a low temperature range.